Seismic surveys, taken at different stages in the life of an oil field, are used to locate pockets of hydrocarbons. These types of surveys are generally referred to as 4-D seismic surveys which incorporate data relating to depth, width, length and time. Problems have been encountered because of the lack of compatibility of surveys performed at various times. Analysis of the data is expensive and often unreliable.
In conducting seismic surveys, energy is released that travels through the earth in the form of vibrations called seismic waves. The seismic waves move in all directions and gradually grow weaker as the distance increases from the source.
There are two kinds of seismic waves. Body waves, the fastest seismic waves, move through the earth. Body waves may be compression waves or shear waves. As the waves pass through the earth, they cause particles of rock to move in different ways. Compressional waves push and pull the rock. Shear waves make rocks move from side to side. Compressional waves can travel through solids, liquids, or gasses, but shear waves can pass only through solids. Compressional waves are the fastest seismic waves and are often referred to as primary waves. Shear waves travel slower and are referred to as secondary waves. Seismograph instruments are equipped with sensors called seismometers that can detect ground motions caused by seismic waves. A seismograph produces wavy lines that reflect the size of seismic waves passing beneath it. The record of the wave, called a seismogram, is printed on paper, film or recording tape or is stored and displayed by computers.
A need exists for a controllable source of seismic energy for use in 4-D seismic mapping which provides compatible data for use in 4-D seismic mapping. Further, a long felt need exists for a source of seismic energy which is controllable so that it can be “fine-tuned” to a particular formation to provide optimum performance in stimulating production of hydrocarbons.
Seismic or elastic wave stimulation is a known technique for enhancing oil recovery from an oil bearing bed as described in “Elastic-Wave Stimulation of Oil Production: A Review of Methods and Results,” Geophysics Vol. 59, No. 6 (June 1994).
Various devices for imparting a shock wave to a well are known in the patented prior art. The Russian Federation Patent No. 1,710,709, for example, discloses a method and apparatus in which an anvil plate is arranged at the bottom of the well and a heavy weight in the form of water filled tubing is repeatedly lifted and dropped onto the anvil plate, thereby imparting vibrations to the oil bed. However, the repeated impact of the weight eventually destroys the bottom of the well. The amount of damage can be minimized by limiting the impact force applied to the anvil plate, but this lowers the power of the elastic waves which reduces efficiency. In addition, the efficiency of this method is limited by a low energy transfer coefficient of the potential energy of the weight into the energy of the elastic waves.
Kostrov USSR Patent No. 1,674,597, granted May 1, 1991 discloses a well hydro-acoustic generator comprising a frame with inlets and outlets, a conical nozzle, a resonant diaphragm and a conical deflector. The conical deflector has a particular angle providing optimum reflection of waves created by the generator and the resonant diaphragm such that the whole energy of the wave is transferred into the direction of the casing wall.
The O Vagin U.S. Pat. No. 5,586,602 discloses a method and apparatus for increasing the effectiveness of shock wave stimulation of oil-bearing formations which includes a pumping unit arranged at the wellhead, a tubing string which extends downwardly into the production casing of the well, a stuffing box arranged on top of the tubing string, a cylinder connected with the bottom of the tubing string, and a plunger which reciprocates up and down within the tubing string and cylinder. As the plunger moves upwardly, the fluid in the tubing string is compressed. At the top of the pumping unit upstroke, the plunger exits the top of the cylinder so that the fluid in the tubing string is discharged into the production casing, thereby generating a shock wave. Although this method achieves higher efficiency than the method described in the Russian Patent No. 1,710,709 discussed above, it is limited in terms of its reliability, effectiveness and efficiency because (1) a cement/bridge plug must be installed to pressurize the borehole, (2) the pressure on the front of the shock wave must be limited due to the low reliability of the stuffing box undergoing a high periodic pressure, (3) additional surface equipment must be used to compensate for liquid leaking through the stuffing box, casing, cement plug, and other equipment, and (4) it is unable to generate a shock wave near the bottom hole for wells having a depth of greater than 800-1000 feet.
Wagner et al U.S. Pat. No. 5,836,389 discloses a conical-shaped diffuser-deflector which is described as being positioned such that as the wave impacts the diffuser-deflector, the wave is partially deflected in an outward direction and at least a portion of the impulse wave continues down-hole, to impact a bridge plug. As the wave deflects off of the bridge plug, weak elastic waves are then maintained in a lower area of the well by a packer and the diffuser-deflector.
Roberts U.S. Pat. No. 5,950,726 discloses well stimulation apparatus which employs an underground casing which provides a sealed vessel in which is supported a tubing string. The casing the tubing string are filled with a working fluid and a pumping unit reciprocates a plunger within the tubing string to cyclically pressurize and depressurize the working fluid for creating elastic wave energy. A hollow conical projector is supported at the lower end of a pump barrel to amplify and direct energy waves. Working fluid substantially fills the tubing assembly and the sealed vessel formed by the well head, the casing and a bridge plug installed above perforations in a live or abandoned well.
Vagin U.S. Pat. No. 5,586,602, Wagner et al U.S. Pat. No. 5,836,389 and Roberts U.S. Pat. No. 5,950,726 disclose processes which require that the well be completely filled with liquid and sealed for forming a closed system.
The present invention was developed to overcome these and other drawbacks of the prior devices by providing an improved method and apparatus for producing an elastic shock wave in a borehole which includes pumping means arranged at the wellhead, a tubing string extending downwardly into the production casing of the well, a hollow cylinder assembly connected with the bottom of the tubing string, and a pair of plungers arranged within the cylinder assembly for compressing liquid contained within the cylinder assembly and discharging the compressed liquid into the production casing, thereby generating a shock wave.